@rhm: Maybe they're not using SL5? SL4 doesn't support hardware acceleration of the video decoding process, so a weaker CPU could choke decoding h.264/VC-1 video. The hardware acceleration box in SL4 just means that it will use your video card for scaling.
The worst part is even if you upgrade to SL5, the video player host needs to enable the option for hw decode. This is why HD Netflix still causes my HTPC CPU to run near 80-90% while streaming. Hopefully Netflix doesn't just abandon their WMC plugin.